Heat-sensitive copy sheet

ABSTRACT

The reaction temperature of a copy sheet containing bis(triphenylphosphine)borohydridocopper(I) as heat-decomposable image-forming component is significantly reduced by incorporating certain nonnitrogenous nickel compounds as catalysts.

United States Patent inventor Joseph A. Wiese, Jr.

Saint Paul, Minn.

Appl. No, 870,536

Filed Oct. 30, 1969 Patented Dec. 21, 1971 Assignee Minnesota Mining andManufacturing Company St. Paul, Minn.

HEAT-SENSITIVE COPY SHEET 10 Claims, No Drawings 117/138.8 A, 117/155UA, 260/41 C, 260/922 lnt.Cl B41m 5/18 Field of Search 1 l7/36.8, 36.9

[56] References Cited I UNlTED STATES PATENTS 2,813,042 11/1957 Gordoneta1... l17/36.8 3,505,093 4/1970 Schultz l l7/36.8

Primary Examiner-Murray Katz Attorney-Kinney, Alexander, Sell, Steldt &Delahunt HEAT-SENSITIVE COPY SHEET This invention relates in a majoraspect to the copying of differentially radiation-absorptive graphicoriginals by thermographic methods involving brief exposure of theoriginal to intense radiant energy while in heat conductive contact witha heat-sensitive copy sheet, and, while not limited thereto, hasparticular reference to novel heat-sensitive copy sheet articles andmethods of making. The heat-sensitive coatings or coated articles arealso useful in thermal telltales or temperature indicator devices.

In U.S. application, Ser. No. 687,126 filed Dec. 1, 1967, now U.S. Pat.No. 3,505,093 there is described a heat-sensitive copy sheet containingas the visibly heat-sensitive material the compoundbis(triphenylphosphine)borohydrid ocopper(l). Localized heating of thecopy sheet under maximum exposure in a thermographic copying machinecauses decomposition of the compound, with formation of a stable darkcolored residue which provides a visible record of the thermographicallyapplied heat pattern.

Many commercially available thermographic copying machines operate atoptimum temperatures which are significantly lower than that requiredfor the sheet materials just described, so that such copy sheets eitherfail to produce a fully developed image, or require that the machine beoperated above its normal rating or at less than its recommended speed,with attendant overheating of machine components and with otherdifficulties. A particular difficulty is the formation of fuzzy orfilled letters in the copy, caused by excessive lateral diffusion of theheat pattern under the highoperating temperatures.

In accordance with the present invention it has now been found possibleto cause permanent visible change in thebis(triphenylphosphine)borohydridocopper(l) coatings at significantlylower temperatures by incorporating catalytically small amounts ofcertain nitrogen-free nickel compounds. Where heat-sensitive copy sheetsare involved, overheating of machine and of printed original is avoided,lamp life is increased, wrinkling of copy is prevented, and sharplydefined copies, with letters free of fuzziness or filling, are obtained.

As described in application, Ser. No. 687,126, the copy sheet willnormally comprise a heat-resistant carrier or backing having a coatingof the visibly heat-sensitive material in a heat-resistant film-formingbinder, although other constructions may also be used. Thin paper andtransparent plastic film are typical carriers. Polyvinyl acetate is apreferred binder, but others, such for example as ethyl cellulose, vinylacetate-vinyl chloride copolymer, and vinylidene chlorideacrylonitrilecopolymer are also useful. The compositions are ordinarily applied fromsolution or suspension in a volatile organic liquid vehicle which isthen removed by evaporation at a temperature below the reactiontemperature. Pigments, fillers,

opacifrers, stabilizers and other additives may be included with thereactant materials, and additional subcoatings or protective surfacecoatings may be added if desired. 7 v

The amount of bis(triphenylphosphine)borohydrid ocopper(l) applied perunit area of coating is at least sufficient to provide a readily visibleimage when locally heated but for reasons of economy should not begreatly in excess of such amount. Good results are obtained at aconcentration of about 1 to 1% grams of the copper compound per squaremeter, i.e., at least about 0.0016 gram mol per sq.m.

Surprisingly small amounts of nickel compound are fully effective insignificantly lowering the imaging temperature, but much larger amountsmay be tolerated. As an example, a sheet containing 1% grams of thecopper compound per sq.m. and showing only a faint darkening at imageareas when subjected to normal thermographic copying procedures is foundto produce sharp images of high density under the same copyingconditions when formulated with but 0.005 gram/sq.m. of nickelacetylacetonate.

Many nonnitrogenous nickel compounds are found similarly to catalyze theimage-forming reaction; but others are ineffective. It has not beenfound possible adequately to distinguish the two classes in terms ofchemical structure.

However it has been found possible to distinguish the two in terms of asimple chemical test which will now be described.

Ten grams of the bis(triphenylphosphine)borohydrid ocopper(l) isdissolved in ml. of chloroform to provide a clear stock solution. Onemilligram of the nickel compound under test is dissolved or dispersed in5 ml. of chloroform. One milliliter of the stock solution is added tothe test mixture and the combination is briefly heated to boiling. Adistinct darkening of the solution indicates that the nickel compound isan effective catalyst.

Tested in the manner described, nickel acetylacetonate produces a darkbrown solution, as does nickel acetate, nickel oxalate, nickeltrifluoromethanesulfonate, nickel formate, nickelocene, and nickeldiethyl dithiocarbamate. A less intense but still distinctly brown coloris obtained with nickel tartrate and with nickel bromide. Each of thesecompounds is useful in lowering the imaging temperature of thebis(triphenylphosphine)borohydridocopper(l) copy sheet, as are nickelstannate, nickel oleate, nickel trifluoroacetylacetonate and nickelhypophosphite. Surprisingly, nickel chloride does not cause a visiblechange; and nickel chloride is found to be ineffective as an additivefor reducing the image-forming temperature of the copy sheet. The sameis true of nickel sulfate.

EXAMPLE A coating composition is prepared by uniformly mixing mg. ofbis(triphenylphosphine)borohydridocopper(l) into 5 grams of a 5 percentsolution of polyvinyl acetate in methylene chloride. The solution iscoated on thin paper, using a knife coater set at an orifice of threemils, and the coating is permitted to dry. This control sheet producesblack images of typewritten correspondence and other originals in thethermographic copying process under maximum temperature conditions butthe images are fuzzy" and the block letters are largely filled.

Another composition is prepared and coated in identical manner exceptfor the addition of 0.5 mg. of nickel acetylacetonate. The copyingmachine is adjusted to an operating condition in which the control sheetproduces only a dim partial image of the original. Under theseconditions the augmented sheet produces complete and sharp images ofhigh image density.

Excellent images are obtained with sheets prepared with compositionscontaining 0.5, 2,5,10 and 20 mg. of the nickel compound. With theaddition of 40 mg. of the compound the mixture becomes gelatinous andcannot be coated.

What is claimed is as follows:

1. An article capable of being heated and having on a surface thereof acoating of a visibly heat-sensitive composition comprising the compoundbis(triphenylphosphine)borohydrid ocopper (l) and a catalytic amountsufficient to significantly depress the decomposition temperature ofsaid compound, of a nonnitrogenous nickel compound characterized by itsability to cause distinct darkening of a solution of bis(triphenylphosphine )borohydridocopper(l) in boiling chloroform.

2. The article of claim 1 wherein said coating includes a polymericfilm-forming binder.

3. The article of claim 2 in the form of a heat-sensitive copy sheetcomprising a thin flexible backing with said coating extending over oneentire major surface.

4. The copy sheet of claim 3 wherein the copper compound is present inan amount of at least about 1 gram per sq. meter and said nickelcompound is present to the extent of about one-half to about 20 parts byweight for each 150 parts of said copper compound.

5. The copy sheet of claim 4 wherein the backing is paper.

6. The copy sheet of claim 4 wherein the backing is thin transparentplastic film.

7. The copy sheet of claim 4 wherein said nickel compound is nickelacetylacetonate.

8. The copy sheet of claim 4 wherein said nickel compound is nickelformate.

9. The copy sheet of claim 4 wherein said nickel compound is nickeltrifluoromethanesulfonate.

10. The copy sheet of claim 4 wherein said nickel compound isnickelocene.

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2. The article of claim 1 wherein said coating includes a polymeric film-forming binder.
 3. The article of claim 2 in the form of a heat-sensitive copy sheet comprising a thin flexible backing with said coating extending over one entire major surface.
 4. The copy sheet of claim 3 wherein the copper compound is present in an amount of at least about 1 gram per sq. meter and said nickel compound is present to the extent of about one-half to about 20 parts by weight for each 150 parts of said copper compound.
 5. The copy sheet of claim 4 wherein the backing is paper.
 6. The copy sheet of claim 4 wherein the backing is thin transparent plastic film.
 7. The copy sheet of claim 4 wherein said nickel compound is nickel acetylacetonate.
 8. The copy sheet of claim 4 wherein said nickel compound is nickel formate.
 9. The copy sheet of claim 4 wherein said nickel compound is nickel trifluoromethanesulfonate.
 10. The copy sheet of claim 4 wherein said nickel compound is nickelocene. 